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The hypothesis that salmonine catches in Lake Ontario are higher at thermal fronts in spring and early summer was tested in 1990 by comparing catches in nonfrontal water and three types of fronts: thermal bar (4°C); spring thermocline (6-8°C); and thermal break (>=9°C). A thermal front in the spring in Lake Ontario is a pronounced temperature cline across the surface of the lake (in this study defined as 0.15Co/min or greater at standard boat speeds) parallel to shore that extends obliquely from the surface toward shore and the bottom. Surface temperature was recorded every 2 min during 45 h of trolling for fish at a standard 3.2-4.8 km/h. Only 20% of the time was spent fishing in thermal fronts, where 35% of the 88 strikes occurred and 37% of the 59 fish were caught. Catch per unit effort (CPUE) for salmonines at thermal fronts was greater than nonfrontal CPUE (P < 0.001 for all strikes; P < 0.05 for fish caught). Catches were better in thermal breaks (P < 0.002) and the spring thermocline (P < 0.05) than in nonfrontal waters. Relative to nonfrontal water, CPUE for coho salmon Oncorhynchus kisutch was greater in the spring thermocline (P < 0.01). Salmonines were caught deeper in nonfrontal waters than in frontal waters (P = 0.014). Chinook salmon O. tshawytscha and lake trout Salvelinus namaycush were caught deeper than were coho salmon and steelhead O. mykiss (P < 0.05). Anglers can effectively enhance their catch of salmonines by fishing the spring thermocline and thermal breaks. These results likely are applicable to other pelagic habitats utilized by salmonines.